Laser pumping assembly

ABSTRACT

A laser pumping assembly with a housing having a removable heat resistant glass liner or shell enclosing a pump and a laser rod comprises removable glass or quartz plates over opposite ends of the shell together with a heat absorbing shim between each end plate and an adjacent heat conductive support plate attached to the housing. The outside surface of each end plate is coated with a dielectric layer which reflects the desired wavelength of pump lamp radiation inwardly of the shell toward the laser rod and transmits undesired wavelengths to the adjacent shim. The shims are held tightly against the end plates and pass heat from the unreflected pump lamp radiation to the respective support plates for dissipation in the cooling system.

This invention was made under a contract with the Department of the AirForce.

RELATED APPLICATION

Ser. No. 866,512 filed Jan. 3, 1978 by Radecki et al, assigned to theassignee of this invention.

BACKGROUND OF THE INVENTION

This invention relates to an improved conductively cooled lamp pumpedrod-type laser.

There has been a continuing problem with conductively cooled solid statelasers in the removal of heat from the optical pump cavity to adjacentparts of the housing while maintaining efficient coupling of pump lampradiation at the desired wavelength to the rod. Prior art lasers of thistype have utilized a dielectric elliptically shaped shell with a coatingthat reflects to the rod only the desired pump wavelengths, suchtechnique being described in an article entitled "Dielectric CavityRaises YAG Cavity Efficiency", by Y. H. Hahn et al published inElectro-Optical Systems Design, February 1975 (Milton S. KiverPublications). Such lasers, however, have depended upon convectioncooling of the shell; that is, the shell is simply exposed to the air orother similar medium. There is no known technique in the prior art forenclosing such a shell in a conductively cooled housing and to do so insuch a manner as to preserve the efficient transfer of heat from theshell to the body.

Another aspect of the above type of conductively cooled lasers is theneed for accessibility of parts for maintenance and repair withoutcompromising the capability of the assembly to withstand vibration andshock forces. In one prior art pump assembly, the interior surfaces ofthe metallic end support plates defined part of the interior of the pumpcavity and were gold plated. The difficulty with this construction isthat unavoidable impurities in the gold plating were "cooked out" in thehigh cavity temperature and contaminated the interior of the cavity.Furthermore, it was difficult to insulate metallic support plates fromthe high voltage lamp energizing leads so as to prevent arcing. Thisinvention is directed to a solution to this problem.

OBJECTS AND SUMMARY OF THE INVENTION

A general object of this invention is the provision of a conductivelycooled laser pumping assembly which efficiently directs pump lampradiation to the laser rod.

A further object is the provision of a laser pump assembly with inertend plates which are heat resistant and free of contaminants.

Still another object is the provision of such end plates which are easyto clean and which prevent arcing between the housing and high voltagelamp circuits.

These and other objects of the invention are achieved with a laserpumping assembly in which opposite ends of the glass cavity liner areclosed by removable glass plates, respectively, held in position by therespective support plates. The exterior of each of these end plates hasa dielectric coating which reflects pump lamp radiation at the desiredwavelength back into the cavity but transmits undesired wavelengths forremoval as heat through the adjacent support plate. Each end plate has acontaminant free inner surface and supports and electrically insulatesthe lamp parts to prevent arcing.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic drawing of a laser system of the typewhich emobdies this invention;

FIG. 2 is an exploded view of a laser pumping assembly embodying thisinvention;

FIG. 3 is a transverse section of the assembly showing the relativepositions of the laser rod, pump lamp and elliptical shell;

FIG. 4 is a greatly enlarged view of the laser rod portion of FIG. 3;

FIG. 5 is an enlarged elevation showing the connection of the laser rodto the heat sink by a spring loaded strap;

FIG. 6 is a view taken on line 6--6 of FIG. 5;

FIG. 7 is an elevation of the pumping assembly; and

FIG. 8 is a greatly enlarged view of a portion of FIG. 7 showing detailsof the end plate connection.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to the drawings, FIG. 1 is a schematic representation of alaser system of the type in which the invention is embodied. The systemcomprises a laser pump lamp 10 having an axis 11 and energized by apower supply 12. Lamp 10, which may be of the alkali metal arc type,illuminates and optically pumps a rod 14 of lasing material, such asneodymium doped yttrium-aluminum-garnet (Nd:YAG), having an axis 15along which the coherent light 16 is generated. Lamp 10 and laser rod 14are mounted within a pumping assembly indicated in broken lines at 17and described in greater detail below.

Coherent light generated by rod 14 typically has a wavelength of 1.064μm and in the system shown by way of example in FIG. 1 is reflected byend mirror 19 back through rod 14, passes through a lens 20 and is againreflected by folding mirror 22 through a frequency doubling crystal 23such as barium sodium niobate (Ba₂ NaNb₅ O₁₅). This crystal doubles thefrequency of a portion of the light beam so as to produce a beam of0.532 μm wavelength along with the remaining undoubled fundamental beamwhich are totally reflected by end mirror 25 back through crystal 23where a second portion of the fundamental is also converted to thesecond harmonic, then to mirror 22 which is coated to transmit the 0.532μm wavelength light while reflecting the 1.064 μm light. The output 26from mirror 22 is therefore green light having a wavelength of 0.532.

Pumping assembly 17 is shown in detail in FIGS. 2 and 3 and comprises asplit housing 28 with two sections 28a and 28b, a unitary ellipticallyshaped transparent shell 30 such as quartz or heat resistant glass,disposed within the housing, laser rod 14' supported on one end of aheat conducting body or heat sink 32 within shell 30, transparent endplates 34 and 35 of quartz or the like over opposite ends of shell 30,and support plates 36 and 37 secured to opposite ends of the housing andover end plates 34 and 35.

Pump lamp 10' extends through end plates 34, 35 and support plates 36,37 for connection at opposite ends to a cathode mount 39 and an anodemount 40, respectively, both mounts being connected to power source 12.

Housing 28 has an elliptically shaped bore 45, see FIG. 3, with focalaxes which are coincident with the axes 11 and 15 of the pump lamp andlaser rod, respectively, when the pumping assembly is fully assembled.The interior of bore 45 is coated with a radiation absorbing substancesuch as black chrome.

Shell 30 is also elliptically shaped and dimensioned so as to fit snuglywithin bore 45 so that the bore and shell have coincident focal axes.The external surface of shell 30 has a multilayer dielectric coatingwhich is designed to transmit lamp wavelengths not used to pump thelaser rod and to reflect lamp wavelengths useful in pumping the laserrod. The unwanted lamp radiation which passes through this coating isabsorbed by the black chrome layer on the housing bore surface and theresulting heat is transmitted by conduction through the housing andconnected to external refrigeration means, not shown. The ellipticalshell with its optically reflective-transmissive multi-layer dielectriccoating is part of the prior art and does not per se constitute thisinvention.

Housing sections 28a and 28b are connected together by flanges 49 and50, respectively, along a plane which contains the focal axes of thebore and shell. The opposite side wall of the housing has an elongatedopening 51 which overlies and registers with a similar opening or slot52 in shell 30. These openings provide access to the interior of thehousing and shell through which the heat sink 32 is inserted, which inturn supports laser rod 14' on the focal axis of the cavity.

Heat sink 32 for laser rod 14' comprises a conductive body havingrecesses for receiving cooling pipes 54 for conductively cooling thestructure in the manner similar to that for housing 28. In accordancewith this invention, rod 14' is mounted on the tapered end 32a of theheat sink body, see FIG. 4, the rod being engaged substantiallytangentially at arcuately spaced points 56 and 57 and being separatedfrom the intermediate parts of the body by a small gap 58. The surfaceof rod 14' between contact points 56 and 57 has an arcuate extent of θ.Radiation shields, not shown, may be provided to cover the portions ofthe heat sink within the elliptical cavity.

Gap 58 contains a solder having a relatively low melting point, i.e.,approximately 10° C., and high heat conductivity in the liquid state. Byway of example, the solder useful for this purpose is anindium-gallium-tin composition made by American Indium Corporation anddesignated as Alloy #51. At room temperature a bond is formed betweenthe rod and heat sink by the solder. The liquid solder wets both theheat sink and the laser rod with excellent adherence and makes a nearperfect thermal contact. In addition, the solder has a low vaporpressure and high reflectivity which are advantageous in the highradiation fields of the pump light. Minimal strain is induced in the rodby use of the solder bond because the laser rod temperature typically isapproximately 10° C. or slightly less when the laser is operating. Thusthe rod and heat sink are capable of expanding and contractingindependently down to 10° C. and no induced stress is transmitted to therod by the solder joint at higher temperatures. This results insignificant improved laser performance because stress inducedbirefringence leading to depolarization of the laser beam in passingthrough the rod is considerably diminished.

Rod 14' is slightly longer than adjacent parts of heat sink 32 and isdisposed so that the rod ends project slightly beyond opposite ends ofthe heat sink. This length differential insures against solder coveringthe ends of the rod. Each of these overhanging rod ends is clamped tothe heat sink by a strap 63, see FIGS. 5 and 6, anchored at one end tothe heat sink body by pin 64 and at the opposite end by a spring 65secured to the body by a pin 66. The tension of spring 65 is adjusted byselection of the location of pin 66 and permits slight relative movementbetween the rod and heat sink due to thermal expansion and contractionwhile preventing undue freedom of movement between these parts thatmight damage the rod. Each clamping mechanism is covered by an end cover67 secured to the sides of the heat sink which protect theanti-reflection coatings on each end of the rod.

Glass shell 30 may lose reflectivity over a period of time due todegradation of dielectric coating or due to contamination of the shell'sinner surface because of the high temperature and radiation environmentsto which it is subjected during operation of the laser. In order toprovide for convenient and quick replacement of shell 30, support plates36 and 37 are removably connected to housing 28. Opposite ends of shell30 are closed by abutting glass end plates 34 and 35 as shown in detailin FIGS. 7 and 8. Each end plate 34, 35 is dimensioned so its peripheralportion extends into a recess 70 in the elliptical surface of thehousing.

In order to further enhance the efficient transmission of pump light tothe laser rod and to remove undesired radiation from the ends of thecavity, the exterior surface of each end plate has the same multilayerdielectric coating that is applied to the external surface of shell 30.The coated outside surface of end plate 34 is indicated at 72 in FIG. 8.Adjacent to and covering these coated end plate surfaces are thinmetallic shims, one of which is shown at 74 in FIG. 8, having insidesurfaces covered with a light absorbing substance such as black chrome.Shims 74 absorb the undesired lamp radiation transmitted through thedielectric coating on the end plates and transmit this as heat tosupport plates 36 and 37 for removal through the cooling system.

Glass end plates 34 and 35 have openings 34a and 35a, respectively, (seeFIG. 2) in which opposite end portions of lamp 35 are supported forextension through larger openings 36a and 37a in support plates 36 and37, respectively. Thus end plates 34 and 35 electrically insulate thelamp from the metallic housing parts and prevent possible arcing. Inaddition, the inner surface of each end plate may readily be cleanedduring maintenance of the assembly.

The subassembly consisting of shell 30, end plates 34 and 35, and shims74 is removable axially from the housing cavity for maintenance andreplacement if necessary. Small axial dimensional differentials betweenthe shell 30 and housing 28 due to operating temperature differences areaccommodated by an axial clearance 75 between shim 74 and support plate36 and by clearance 78 within gap 70. Clearance 75 is held to a minimumin order to provide efficient heat transfer from the shim to the endplate and is preferably 0.001-0.005 inches. In order to hold thesesubassembly parts tightly together, a plurality of retainer springs, oneof which is shown at 76 in FIGS. 2 and 8, are mounted in appropriateaxially parallel recesses 77 in support plate 36 and abut against shim74. The shell-end plate-shim subassembly is thus axially resilientlyheld against support plate 37 so as to make it vibration resistant whileaccommodating thermal expansion and manufacturing tolerances.

What is claimed is:
 1. In a conductively cooled laser pumping assemblycomprising a split housing having an elliptically shaped bore with firstand second focal axes, said sections being connected symmetrically abouta plane containing said axes and being spaced apart adjacent to saidfirst axis whereby to define an elongated opening in said housing, aunitary elliptically shaped transparent shell snugly fitted in said boreand having focal axes coincident with said first and second axes,respectively, said shell also having an elongated slot aligned with saidopening in said housing, lamp means supported within said shell andhaving an axis coincident with said second axis, said lamp means beingadapted to radiate noncoherent light at a desired wavelength λ₁ and atundesired wavelengths λ_(u), said shell being covered by a thin layer ofa dielectric substance, said layer being highly reflective at λ₁ andhighly transmissive at λ_(u) and a cylindrical rod of lasing materialsupported within said shell with the rod axis coincident with saidsecond axis, the improvement comprisingend plates engaging oppositeends, respectively, of said shell, said end plates being formed of amaterial substantially transparent to radiation at wavelengths λ₁ andλ_(u), each of said plates having a coating of said dielectric substanceon the exterior surface thereof whereby to reflect radiation at λ₁ intosaid shell and to transmit radiation at λ_(u), and heat conductivesupport plates secured to said housing at opposite ends, respectively,of said bore and over said end plates, respectively, said support platesbeing adapted to remove heat produced by said λ_(u) radiation.